Hitherto, in a fuel cell system, adsorption of oxygen into a catalyst layer of a fuel cell in operation causes a drop in an output voltage of the fuel cell. In such a case, a process for activating the catalyst layer of a fuel cell stack (i.e., reduction process) is carried out by interrupting the supply of oxygen to the fuel cell and decreasing the power generation voltage of the fuel cell down to a reduced zone of the catalyst layer.
As a technique related to such a catalyst activation process of a fuel cell, there has been proposed, for example, a technique wherein a low-voltage battery is used for an auxiliary device constituting a hybrid fuel cell, and surplus power which increases as voltage decreases during the catalyst activation process of the fuel cell is charged in the battery so as to obviate wasting surplus power (the publication of Japanese Patent Application Laid-Open No. 2005-346979).